Crushed monsters block player movement

In early versions of Doom, the player would not be able to step over corpses if they were killed by a fast crushing ceiling.

Monsters that were killed first and then crushed would properly allow players to walk "through" the corpse. However, if the monster was killed by a crushing ceiling fast enough that no death animation played, and the monster immediately became a pool of blood, players and other monsters would not be able to cross over the pool of blood and would instead have to walk around it. This could potentially render certain levels unfinishable if the crushed monsters blocked the path forward.

The fix that id Software applied to this problem (setting the height and radius of the crushed monster to zero) later caused ghost monsters to be spawned by arch-viles.